Cardiovascular Effects of Adenosine Derivatives in Conscious Normotensive Rats

Abstract

All adenosine receptor agonists, regardless of their A₁/A₂ selectivity ratio, dose dependently reduced blood pressure (MAP) whereas their effects on heart rate (HR) and plasma renin activity (PRA) depended on their receptor subtype selectivity. Thus an adenosine receptor agonist with an optimal A₁- and A₂- receptor selectivity (no increase in HR and PRA) and which does not penetrate the brain, might be a useful antihypertensive drug.     

Strategically Functionalized Adenosines: Agonists for Adenosine Receptors

Abstract

The syntheses of three classes of adenosine analogues involving cyclosubstitution at the 6-position and functionalization at the 2-position are reported. The target molecules synthesized are stable with respect to hydrolytic deamination by mammalian adenosine deaminase, and, because of major structural changes at the 2- and 6-positions, these compounds are expected to be poor phosphorylation substrates for the kinases. Adenosine receptor binding data reveal that several of the compounds synthesized show excellent A₁ receptor affinity and A₂/A₁ selectivity.     

Effects of Adenosine Receptor Subtypes on Hippocampal Extracellular Serotonin Level and Serotonin Reuptake Activity

Abstract: To clarify the effects of adenosine receptor subtypes (A₁, A₂, and A₃) on hippocampal serotoninergic function, hippocampal extracellular serotonin (5-HT) levels were determined by in vivo microdialysis in freely moving rats under various conditions. Both adenosine and an adenosine A₁ receptor agonist, 2-chloro-N⁶-cyclopentyladenosine, decreased extracellular 5-HT levels, whereas an adenosine A₁ receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (CPT), and caffeine increased these levels. A selective A_2A receptor agonist (CGS-21680), an adenosine A₂ receptor agonist (PD-125944), an adenosine A₂ receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX), and an adenosine A₃ receptor agonist, N⁶-2-(4-aminophenyl)ethyladenosine (APNEA), did not affect extracellular 5-HT levels. When the adenosine A₁ receptor was blocked by CPT, the hippocampal extracellular 5-HT level was increased by adenosine, CGS-21680, and PD-125944, and decreased by caffeine, DMPX, and APNEA. When both adenosine A₁ and A₂ receptors were blocked by CPT and DMPX, the extracellular 5-HT level was decreased by adenosine, caffeine, and APNEA. The hippocampal extracellular 5-HT level was not affected by administration of APNEA alone, but was decreased by this agent when the adenosine A₁ receptor was blocked, irrespective of whether the adenosine A₂ receptor was functional. These inhibitory effects of adenosine, caffeine, and APNEA on extracellular 5-HT levels, during both adenosine A₁ and A₂ receptor blockade, were inhibited by selective 5-HT reuptake inhibitors. These results indicate that the stimulatory effects of the adenosine A₂ receptor and the inhibitory effects of the A₃ receptor on hippocampal extracellular 5-HT levels are masked by the inhibitory effects of the adenosine A₁ receptor.     

Deaza-analogues of Adenosine and 2-Chloroadenosine as Agonists of Adenosine Receptors

Abstract

A variety of adenosine analogues have been recently evaluated in order Lo find more potent and selective agonists on adenosine receptors. The most potent adenosine analogues acting on A₁ receptor, a high affinity receptor inhibitory to adenylate cyclase, are N⁶-substituted compounds. So ⁶-cyclohexyladenosine (CHA) and ⁶-L-phenylisopropyladenosine (L-PIA) are extremely potent agonists on A₂ receptor, whereas they are relatively weak agonists on A receptor, a lower affinity receptor which is stirnulatory to cyclase, and they have no effect on the adenosine P site.     

Imidazo[1,2-α]pyridines possess adenosine A1 receptor affinity for the potential treatment of cognition in neurological disorders

Previous research has shown that bicyclic 6:5-fused heteroaromatic compounds with two N-atoms have variable degrees of adenosine A₁ receptor antagonistic activity. Prompted by this imidazo[1,2-α]pyridine analogues were synthesized and evaluated for their adenosine A₁ and A_2A receptor affinity via radioligand binding studies and subjected to a GTP shift assay to determine its adenosine A₁ receptor agonistic or antagonistic functionality. Imidazo[1,2-α]pyridine, the parent scaffold, was found devoid of affinity for the adenosine A₁ and A_2A receptors. The influence of substitution on position C2 showed no improvement for either adenosine A₁ or A_2A receptor affinity. The addition of an amino or a cyclohexylamino group to position C3 also showed no improvement of adenosine A₁ or A_2A receptor affinity. Surprisingly para-substitution on the phenyl ring at position C2 in combination with a cyclohexylamino group at position C3 led to adenosine A₁ receptor affinity in the low micromolar range with compound 4d showing: (1) the highest affinity for the adenosine A₁ receptor with a K_i value of 2.06 µM and (2) adenosine A₁ receptor antagonistic properties. This pilot study concludes that para-substituted 3-cyclohexylamino-2-phenyl-imidazo[1,2-α]pyridine analogues represent an interesting scaffold to investigate further structure-activity relationships in the design of novel imidazo[1,2-α]pyridine-based adenosine A₁ receptor antagonists for the treatment of neurodegenerative disorders.     

5-Substituted 2-benzylidene-1-tetralone analogues as A1 and/or A2A antagonists for the potential treatment of neurological conditions

Adenosine A₁ and A_2A receptors are attracting great interest as drug targets for their role in cognitive and motor deficits, respectively. Antagonism of both these adenosine receptors may offer therapeutic benefits in complex neurological diseases, such as Alzheimer’s and Parkinson’s disease. The aim of this study was to explore the affinity and selectivity of 2-benzylidene-1-tetralone derivatives as adenosine A₁ and A_2A receptor antagonists. Several 5-hydroxy substituted 2-benzylidene-1-tetralone analogues with substituents on ring B were synthesized and assessed as antagonists of the adenosine A₁ and A_2A receptors via radioligand binding assays. The results indicated that hydroxy substitution in the meta and para position of phenyl ring B, displayed the highest selectivity and affinity for the adenosine A₁ receptor with K_i values in the low micromolar range. Replacement of ring B with a 2-amino-pyrimidine moiety led to compound 12 with an increase of affinity and selectivity for the adenosine A_2A receptor. These substitution patterns led to enhanced adenosine A₁ and A_2A receptor binding affinity. The para-substituted 5-hydroxy analogue 3 behaved as an adenosine A₁ receptor antagonists in a GTP shift assay performed with rat whole brain membranes expressing adenosine A₁ receptors. In conclusion, compounds 3 and 12, showed the best adenosine A₁ and A_2A receptor affinity respectively, and therefore represent novel adenosine receptor antagonists that may have potential with further structural modifications as drug candidates for neurological disorders.     

Synthesis and Cardiac Pharmacology of 2-(AR)Alkoxyadenosines

Abstract

Certain relatively large 2-(ar)alkoxy substituents selectively raise the agonist potency of adenosine at the A₂ receptor of coronary artery while lowering activity at the A₁ receptor of AV node.     

Characteristics of Adenosine-Mediated Inhibition of Tachykinin Release from Perifused Myenteric Plexus Synaptosomes

Abstract

Adenosine receptor agonists were shown to inhibit evoked release of the tachykinins Substance P and Neurokinin-A from perifused myenteric synaptosomes. The potencies of selective A₁ and A₂ agonists are consistent with activity at adenosine A₁ receptors located on the nerve endings.     

Characterization of the Binding of a Novel Non-Xanthine Adenosine Antagonist Radioligand, [3H]CGS 15943, to Multiple Affinity States of the Adenosine A1 Receptor in the Rat Cortex

Abstract

The use of xanthine adenosine receptor antagonists such as 1,3-dipropyl-8-phenylxanthine (DPX) as radioligands for the characterization of adenosine receptor Pharmacology have been limited by their high lipophilicity, low specific activity, and their general lack of selectivity and affinity for adenosine receptors. Recent attempts to address the technical problems associated with this class of compounds has resulted in the development of several xanthine derivatives (e.g. the functionalized xanthine congeners [³H]XCC and [³H]XAC², and [³H]CPX³) which bind with high and selective affinity to the adenosine A₁ receptor subtype. Based on efforts to optimize non-xanthine adenosine receptor antagonists, CGS 15943, a derivative of the pyrazoloquinazoline benzodiazepine receptor inverse agonist CGS 8216⁵, represents the first reported non-xanthine structure that potently blocks adenosine receptors⁶. CGS 15943 has nanomolar affinity for both A₁ and A₂ receptor subtypes⁶. However, in contrast to many of the xanthine adenosine receptor antagonists, CGS 15943 is not a phosphodiesterase inhibitor and does not interact with adenosine transporter sites⁶. This compound is a potent and selective adenosine receptor antagonist in vivo ⁷ with a solubility/affinity ratio of greater than 1000⁷. In the present studies, the binding of [³H]CGS 15943 to the adenosine A₁ receptor was characterized.     

New 2,6,9-trisubstituted adenines as adenosine receptor antagonists: a preliminary SAR profile

A new series of 2,6,9-trisubstituted adenines (5–14) have been prepared and evaluated in radioligand binding studies for their affinity at the human A₁, A_2A and A₃ adenosine receptors and in adenylyl cyclase experiments for their potency at the human A_2B subtype. From this preliminary study the conclusion can be drawn that introduction of bulky chains at the N ⁶ position of 9-propyladenine significantly increased binding affinity at the human A₁ and A₃ adenosine receptors, while the presence of a chlorine atom at the 2 position resulted in a not univocal effect, depending on the receptor subtype and/or on the substituent present in the N ⁶ position. However, in all cases, the presence in the 2 position of a chlorine atom favoured the interaction with the A_2A subtype. These results demonstrated that, although the synthesized compounds were found to be quite inactive at the human A_2B subtype, adenine is a useful template for further development of simplified adenosine receptor antagonists with distinct receptor selectivity profiles.     

The Synthesis and Biological activity of a Highly Selective Adenosine A2a. Receptor Agonist

Abstract

Three novel nucleosides 1, 2, and 3 were prepared that contained side chains at the 2-position of adenosine. Compound 1 was shown to be the most selective A_2a receptor agonist reported to date having an A₁/A₂ ratio of 2400. In addition, compound 1 was shown to reduce blood pressure in rats and dogs with only minimal effects on heart rate.     

C2-substituted quinazolinone derivatives exhibit A1 and/or A2A adenosine receptor affinities in the low micromolar range

Antagonists of the adenosine receptors (A₁ and A_2A subtypes) are widely researched as potential drug candidates for their role in Parkinson’s disease-related cognitive deficits (A₁ subtype), motor dysfunction (A_2A subtype) and to exhibit neuroprotective properties (A_2A subtype). Previously the benzo-α-pyrone based derivative, 3-phenyl-1H-2-benzopyran-1-one, was found to display both A₁ and A_2A adenosine receptor affinity in the low micromolar range. Prompted by this, the α-pyrone core was structurally modified to explore related benzoxazinone and quinazolinone homologues previously unknown as adenosine receptor antagonists. Overall, the C2-substituted quinazolinone analogues displayed superior A₁ and A_2A adenosine receptor affinity over their C2-substituted benzoxazinone homologues. The benzoxazinones were devoid of A_2A adenosine receptor binding, with only two compounds displaying A₁ adenosine receptor affinity. In turn, the quinazolinones displayed varying degrees of affinity (low micromolar range) towards the A₁ and A_2A adenosine receptor subtypes. The highest A₁ adenosine receptor affinity and selectivity were favoured by methyl para-substitution of phenyl ring B (A₁K_i = 2.50 μM). On the other hand, 3,4-dimethoxy substitution of phenyl ring B afforded the best A_2A adenosine receptor binding (A_2AK_i = 2.81 μM) among the quinazolinones investigated. In conclusion, the quinazolinones are ideal lead compounds for further structural optimization to gain improved adenosine receptor affinity, which may find therapeutic relevance in Parkinson’s disease-associated cognitive deficits and motor dysfunctions as well as exerting neuroprotective properties.     

A1 receptors mediate adenosine inhibitory effects in mouse ileum via activation of potassium channels

AimsWe investigated the effects induced by exogenous adenosine on the spontaneous contractile activity of the longitudinal muscle of a mouse ileum, the receptor subtypes activated, the involvement of enteric nerves and whether opening of K⁺ channels was a downstream event leading to the observed effects.Main methodsMechanical responses of the mouse ileal longitudinal muscle to adenosine were examined in vitro as changes in isometric tension.Key findingsAdenosine caused a concentration-dependent reduction of the spontaneous contraction amplitude of the ileal longitudinal muscle up to its complete disappearance. This effect induced was markedly reduced by an A₁ receptor antagonist, but not by A₂ and A₃ receptor antagonists and mimicked only by the A₁ receptor agonist. Adenosine uptake inhibitors did not change adenosine potency. A₁ receptor expression was detected at the smooth muscle level. Adenosine responses were insensitive to tetrodotoxin, atropine or nitric oxide synthase inhibitor. Tetraethylammonium and iberiotoxin, BK_Ca channel blockers, significantly reduced adenosine effects, whilst 4-aminopyridine, a K_v blocker, apamin, a small conductance Ca²⁺-activated K⁺ (SK_Ca) channel blocker, charybdotoxin, an intermediate conductance Ca²⁺-activated K⁺ (IK_Ca) and BK_Ca channel blocker, or glibenclamide, an ATP-sensitive K⁺ channel blocker, had no effects. The combination of apamin plus iberiotoxin caused a reduction of the purinergic effects greater than iberiotoxin alone.SignificanceAdenosine acts as an inhibitory modulator of the contractility of mouse ileal longitudinal muscle through postjunctional A₁ receptors, which in turn would induce opening of BK_Ca and SK_Ca potassium channels. This study would provide new insight in the pharmacology of purinergic receptors involved in the modulation of the gastrointestinal contractility.     

Adenosine Receptor Classification: Quo Vadimus?

Abstract

Considerable progress has been made in our understanding of the diversity of adenosine receptors during the last decade, with the cloning of the orphan receptors RDC7 and RDC8 (1), and their subsequent characterisation as canine A₁ and A₂ receptors respectively (2,3), in the late 1980s. The principal objective of this review is to produce an integrated view of adenosine receptor classification, using the important observations from studies of molecular biology, receptor binding characteristics and functional pharmacology.     

Synthesis and Receptor Affinity of Polysubstituted Adenosines

Abstract

In a search for potent and selective adenosine agonists it has been found that 2-hexynyladenosine-5′-N-ethyluronamide (HENECA) displays high affinity at rat A_2A receptor combined with a good A_2A vs A₁ selectivity. The finding that HENECA shows good affinity also for A₃ receptors prompted us to investigate the effect of various substituents in different positions of this molecule.     

Adenosine Receptors in Isolated Tissue Preparations

Abstract

Adenosine receptors in isolated tissues from guinea-pigs have been investigated using agonists and antagonists. The receptor mediating decreases in rate and force of contraction of the atria is of the A₁ sub-type. Responses of smooth muscle preparations to adenosine and its analogues are complex involving relaxation mediated by both A₂ receptors an by non-receptor mechanisms. Contractions mediated by an A₁ receptor can also be detected.     

Adenosine and 2-Chloroadenosine Deaza-Analogues as Adenosine Receptor Agonists1

Abstract

Several deaza-analogues of adenosine and 2-chloro-adenosine have been examined for their adenosine receptor affinity. It was found that the relative contribution of the nitrogen atoms of the purine moiety to binding at A₁ rat brain adenosine receptor, follows the order N⁷ > N³ > N¹. The affinity of the adenosine analogues for the adenosine rat brain receptor was besides compared with their activity as inhibitors of platelet aggregation. A synthesis of 2-chloro-1-deazaadenosine by two alternative routes starting from 7-nitroimidazo[4,5-b]pyridine-4-oxide is also reported.     

Xanthine-7-Ribosides as Adenosine Al Receptor Antagonists: Further Evidence for Adenosine's Anti Mode of Binding

Abstract

The synthesis and A₁ adenosine receptor affinity of some xanthine-7-ribosides is described. It appears that these compounds are A, receptor antagonists. The orientation of the ribose moiety, as determined by ′H-NMR spectroscopy and theoretical chemical calculations, is compared with the orientation of the ribose in the agonist adenosine. Implications for the syn vs anti modes of binding to the receptor are discussed.     

A1 and A2 Adenosine Receptors Involvement in Controlling Purine and Acetylcholine Release From Rat Hippocampal Slices

Abstract

The different role of presynaptic A₁ and A₂ adenosine receptor subtypes on a possible autoregulation of endogenous purine outflow as well as on the ACh release, simultaneously assayed, was evaluated in rat hippocampal slices, at rest and under a field electrical stimulation.     

Adenosine A1 receptors contribute to mitochondria vulnerability to pro-oxidant stressors

A₁ adenosine receptors are highly expressed in the central nervous system. Mitochondrial function is a major player in adenosine receptors-mediated effects. Here, by using mice with genetic deletion of the A₁ receptor, we addressed the existence of a relationship between mitochondria functions and adenosine A₁ receptor. Mitochondrial functions and effects of MPP⁺ in primary mixed cultures are influenced by the presence of the A₁ receptor, demonstrating, for the first time, the mitochondrial localization of the adenosine A₁ receptor and suggesting a role for this receptor as a mitochondrial vulnerability factor.